Effects of thermal annealing on the macroscopic dimension and lattice parameter of heavily neutron-irradiated silicon carbide

Reaction-sintered β-SiC specimens were neutron-irradiated in fast breeder reactors to fluences from 3.0x10²⁴ to 1.7x10²⁷ n/m² (E>0.1 MeV) at temperatures from 370 to 620°C. Irradiation to fluences above 4.8x10²⁶ n/m² caused swelling (growth) that was significantly greater in respect of macroscopic dimension than of lattice parameter. These highly irradiated specimens also showed significant broadening of their X-ray line profiles, accompanied by generation of interstitial dislocation loops. Upon annealing, all specimens, irrespectively of the fluence to which they had been irradiated, tended toward recovery of their swelled macroscopic dimension and lattice parameter. This tendency was initiated when the annealing temperature exceeded that of irradiation, with the percentage of residual swelling lowering roughly in proportion to annealing temperature. The more highly irradiated specimens, in which the macroscopic dimension had swelled significantly more than the lattice parameter, lowered their percentages of residual swelling while maintaining a constant difference between them, as the annealing temperature was raised from that of irradiation to 1,200°C. In a certain range of higher annealing temperatures, the lattice parameter fell below the pre-irradiation level. In the range below 1,200°C, the broadened X-ray line profile indicated no tendency to recover. Microstructural observations have indicated that the size and the number density of dislocation loops remain unchanged up to 1,000°C. The recovery of swelled macroscopic dimension and lattice parameter by annealing is discussed in terms of the recombination of point defects, which occurs even in the specimens irradiated to 1.7x10²⁷ n/m² fluence.